1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process, such as an electrostatic copier or a laser printer. More particularly, the present invention relates to an image forming apparatus involving a so-called fixing step for fixing a toner image onto a recording sheet such as a paper.
2. Description of the Related Art
A conventional image forming apparatus of this type has a structure as shown in FIG. 1. An image forming apparatus shown in FIG. 1 has a photoconductive drum 1, charging means (charging unit) 2, exposing means 3, developing means 4, transferring means 5, cleaning means 7, and fixing means 8 to form an image on a recording sheet 6. A typical image forming process will be shown below.
(1) The charging means 2 charges a surface of the photoconductor 1 to a desired potential.
(2) The exposing means 3 exposes the photoconductor 1 to form a latent electrostatic image corresponding to a desired image on the photoconductor.
(3) The developing means 4 develops the latent electrostatic image formed by the exposing means 3 with a toner and thereby forms a toner image on the photoconductor 1.
(4) The transferring means 5 transfers the toner image from the photoconductor to the recording sheet 6 carried by carrying means not shown.
(5) The cleaning means 7 cleans, from the photoconductor, the toner remaining thereon without being transferred to the recording sheet 6.
(6) The recording sheet 6 is carried to the fixing means 8.
(7) The fixing means 8 heats the toner (recording sheet 6) to fix it onto the recording sheet.
The image forming apparatus forms a desired image on the recording sheet 6 by rotating the photoconductive drum in the direction indicated by the arrow in FIG. 1 and repeating the steps (1) to (7).
In general, the photoconductive drum 1 is formed by coating a photoconductor on a surface of a conductor. The mainstream of the photoconductor has been a so-called organic photoconductor. As the photoconductor, a multilayer type (multilayer organic photoconductor) composed of a so-called charge generating layer and a charge transporting layer stacked on a conductive base has been used in most cases because of the high durability of the charge transporting layer. There has also appeared a photoconductor having a protection layer in addition to the charge generating layer and the charge transporting layer, which is provided to enhance the durability of the charge transporting layer.
There has been known that, to provide such an image forming apparatus capable of image reproduction at even a spatial frequency at which a development field is high, the film thickness of a photoconductor should be reduced (thinned) (Basic and Applied Electrophotographic Techniques, pp.150-151, Corona Publishing Co., Ltd.).
However, a photoconductor small in film thickness is low in durability against cleaning-induced abrasion, flaws, or the like. If a charging step and an exposing step are performed repeatedly, the photoconductor small in film thickness deteriorates fast. Briefly, the durability of a photoconductor is reduced significantly if the film thickness thereof is reduced for the formation of a high-quality image. If the film thickness of the photoconductor is increased for the enhanced durability thereof, on the other hand, only a low-quality image is obtainable. To eliminate the tradeoff, it has been requested to satisfy the two requirements of enhanced durability of a photoconductor and high-quality image formation.
In a conventional multilayer organic photoconductor, polycarbonate has been used commonly as a binder resin in a charge transporting layer. In such a photoconductor, the film thickness of the charge transporting layer has normally been adjusted to a range of 20 xcexcm to 30 xcexcm. This indicates that a higher priority has been given to the high durability of the photoconductive than to image qualities.
When the present inventors formed an image at a resolution of 1200 dpi or more which had been known to be necessary for the discrimination of character images (font) by using an image forming apparatus having a multilayer photoconductor in which a charge transporting layer has a film thickness of 20 xcexcm to 30 xcexcm, the image forming apparatus could not reproduce an image at a so-called high spatial frequency such as a single isolated dot or a 1-dot line. This indicates that the conventional image forming apparatus cannot perform through outputting of input images such as so-called bit map images due to unsatisfactory reproduction of a single isolated dot or a 1-dot line. In other words, it is not until an input image has undergone complicated image processing steps that an image can be formed.
If the resolution is adjusted to 600 dpi or 400 dpi, a single isolated dot or a 1-dot line can be reproduced. However, since the single isolated dot or 1-dot line is increased in size, only a coarse image is formed. In an image including an oblique line, a reduction in resolution aggravates a so-called jaggy and degrades image qualities.
The present inventors also performed writing of image data processed with a halftoning operation using the number of lines of 200 lpi or more by using an image forming apparatus having a photoconductor including a charge transporting layer with a film thickness of about 20 xcexcm to 30 xcexcm. The resulting image was extremely low in tone. This proved that an image which needs tonic representation, such as a photographic image, could not be formed satisfactorily. This also proved that so-called banding was likely to occur and only an image with much noise was obtained.
When a halftoning operation was performed by using the number of lines less than 200 lpi, a sufficient tone was achieved. However, a dither texture was visually observed and a fine-texture image could not be obtained.
The present invention has been achieved in view of the aforementioned problems. It is therefore an object of the present invention to provide an image forming apparatus capable of providing a high-quality image by adjusting the diameter of a beam for forming a latent image which is applied to the photoconductor to 35 xcexcm or less and combining a condition placed on the carrier mobility of the charge transporting layer of a photoconductor or a condition placed on the transmittance of a protection layer provided on the photoconductor with a condition placed on the film thicknesses of the protection layer and the charge transporting layer.
A first image forming apparatus according to the present invention is an image forming apparatus having at least: a photoconductor; charging means; and optical writing means for performing an optical write operation with respect to the photoconductor to form a latent electrostatic image thereon, the apparatus using an electrophotographic process in which a resolution of the optical write operation is 1200 dpi or more, the optical writing means performing the optical write operation by using a laser beam with a diameter of 35 xcexcm or less, the photoconductor being provided with at least a charge generating layer containing a charge generating substance and a charge transporting layer containing a charge transporting substance, the charge transporting layer having a carrier mobility of 1xc3x9710xe2x88x925 cm2xc2x7Vxe2x88x921xc2x7secxe2x88x921 or more under an electric field of 3xc3x97105 Vxc2x7cmxe2x88x921.
The image forming apparatus having such a structure is capable of reproducing an image at a high spatial frequency such as a single isolated dot or a 1-dot line and performing through outputting of bit map images or the like. Even in an image including an oblique line, therefore, a so-called jaggy does not occur. In character images also, various fonts can be discriminated.
A second image forming apparatus according to the present invention is an image forming apparatus having at least: a photoconductor; charging means; optical writing means for performing an optical write operation with respect to the photoconductor to form a latent electrostatic image thereon; and image processing means for performing a halftoning operation with respect to an input image, the apparatus using an electrophotographic process which allows the optical writing means to perform the optical write operation based on image data obtained by performing the halftoning operation using a number of lines 200 lpi or more with respect to the input image, the optical writing means performing the optical write operation by using a laser beam with a diameter of 35 xcexcm or less, the photoconductor being provided with at least a charge generating layer containing a charge generating substance and a charge transporting layer containing a charge transporting substance, the charge transporting layer having a carrier mobility of 1xc3x9710xe2x88x925 cm2xc2x7Vxe2x88x921xc2x7secxe2x88x921 or more under an electric field of 3xc3x97105xc2x7Vxc2x7cmxe2x88x921.
In the image forming apparatus having such a structure, tone is improved so that even an image which needs tonal representation, such as a photographic image, is reproduced satisfactorily. As a result, it becomes extremely difficult to visually observe a dither texture which occurs with a smaller number of lines of 200 lpi or less. In addition, the occurrence of banding is minimized.
The present inventors found that, if at least one compound having a triarylamine structure is contained in the charge transporting layer, the charge transporting layer is allowed to have the aforementioned carrier mobility. This may be because, since a compound having a triarylamine structure has a high carrier mobility, it imparts the aforementioned property to the charge transporting layer.
It was also proved that at least compounds expressed by the following structural formulae (A-I) to (A-VI) as compounds each containing a triarylamine structure are excellent in miscibility with a binder resin contained in the charge transporting layer and capable of enhancing the resistance of the photoconductor to oxidizing gas and the optical stability thereof. 
(where R1, R3, and R4 may be the same or different and each independently represents a hydrogen atom, an amino group, an alkoxy group, a thioalkoxy group, an aryloxy group, a methylenedioxy group, a substituted or unsubstituted alkyl group, a halogen atom, or a substituted or unsubstituted aryl group; R2 represents a hydrogen atom, an alkoxy group, a substituted or unsubstituted alkyl group or halogen, except for a combination in which each of R1, R2, R3, and R4 is a hydrogen atom; and k, l, m, and n are each independently 1, 2, 3, or 4). 
(where Ar1 and Ar2 may be the same or different and each independently represents a substituted or unsubstituted aryl group or a substituted or unsubstituted heterocyclic group; R6, R7, and R5 may be the same or different and each independently represents a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heterocyclic group, of which R7 and R6 may be combined to form a ring; and Ar3 represents a substituted or unsubstituted allylene group). 
(where R10, R11, and R12 may be the same or different and each independently represents a hydrogen atom, a halogen atom, a nitro group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkoxy group, or a substituted or unsubstituted aryl group; R8 and R9 may be the same or different and each independently represents a hydrogen atom, an alkoxycarbonyl group, a substituted or unsubstituted alkyl group, or a substituted or unsubstituted aryl group; W represents a hydrogen atom, a substituted or unsubstituted alkyl group, a phenylthio group, a divalent chain unsaturated hydrocarbon group, a monovalent or divalent and substituted or unsubstituted carbocyclic aromatic group, or a monovalent or divalent and substituted or unsubstituted heterocyclic group; j represents an integer of 1 to 5; f represents an integer of 1 to 4; g represents an integer of 1 or 2; h represents an integer of 1 or 2; and i represents an integer of 1 to 3). 
(where Ar4 represents a condensed polycyclic hydrocarbon group having 18 or less carbon atoms; and R13 and R14 may be the same or different and each independently represents a hydrogen atom, a halogen atom, a substituted or unsubstituted alkyl group, an alkoxy group, or a substituted or unsubstituted phenyl group). 
(where R15 and R16 may be the same or different and each independently represents a lower alkyl group, a lower alkoxy group, or a halogen atom; p and q each independently represents an integer of 1 to 4; and R17 and R18 may be the same or different and each independently represents a hydrogen atom, a lower alkyl group, a lower alkoxy group, or a halogen atom). 
(where R19, R20, R21, and R22 may be the same or different and each independently represents a hydrogen atom, an alkyl group which may have a substituent, an alkoxy group, an allyl group, an aryl group, or a halogen atom; and R23 and R24 may be the same or different and each independently represents a hydrogen atom, an alkyl group, an alkoxy group, a halogen atom, an amino group, an N-substituted amino group, an allyl group, or an aryl group).
A content of the charge transporting substance in the charge transporting layer is adjusted appropriately to 40% by weight or more, preferably 50% by weight or more with respect to a total amount of the charge transporting layer. If the content of the charge transporting substance is increased, the carrier mobility of the charge transporting layer is increased. The present inventors found that, if the content of the charge transporting substance was adjusted to 40% by weight or more with respect to a total amount of the charge transporting layer, extremely excellent image qualities were obtained, while the temperature dependence was reduced and degradation of image qualities by environments (low-temperature/low-humidity or high-temperature/high-humidity environments) was reduced. The present inventors also found that, if the content of the charge transporting substance was adjusted to 50% by weight or more, the dependence of the carrier mobility on the intensity of an electric field was reduced.
In the photoconductor, a film thickness of the charge transporting layer is preferably 20 xcexcm or less. It was found that, if the aforementioned structure was used, both high image qualities and the high durability of the photoconductor were achievable and, if the film thickness of the charge transporting layer (CT film thickness) was 20 xcexcm or less, a sufficient durability was obtainable and an extremely high-quality image was obtainable. Since the CT film thickness is small, cost for manufacturing a photoconductive drum can be reduced and a coated film may be excellently uniform. If such a structure is used, the photoconductor is preferably provided with a protection layer for the high durability (for the prevention of a shorter life) of the photoconductive drum.
A third image forming apparatus according to the present invention is an image forming apparatus having: charging means; a photoconductor; and optical writing means to form an image at a resolution of at least 1200 dpi, the optical writing means emitting a laser beam with a diameter of 35 xcexcm or less, the photoconductor having at least a charge generating layer containing a charge generating substance and a charge transporting layer containing a charge transporting substance which are provided on a conductive support, a protection layer being disposed closer to a surface of the photoconductor than the charge generating layer and the charge transporting layer to have a transmittance of 90% or more with respect to the laser beam, a total film thickness of the charge transporting layer and the protection layer being 20 xcexcm or less.
A fourth image forming apparatus according to the present invention is an image forming apparatus having: image processing means for performing a halftoning operation with respect to an input image; charging means; a photoconductor; and optical writing means, the image processing means performing the halftoning operation with respect to at least the input image by using a number of lines of 200 lpi or more, the optical writing means emitting a laser beam with a diameter of 35 xcexcm or less, the photoconductor having at least a charge generating layer containing a charge generating substance and a charge transporting layer containing a charge transporting substance which are provided on a conductive support, a protection layer being disposed closer to a surface of the photoconductor than the charge generating layer and the charge transporting layer to have a transmittance of 90% or more with respect to the laser beam, a total film thickness of the charge transporting layer and the protection layer being 20 xcexcm or less.
Preferably, the protection layer contains a filler, a charge transporting substance, and/or a binder resin.
Preferably, the filler has a refractive index in a range of 1.0 to 2.0 in terms of providing a high transmittance and a satisfactory image.
Preferably, the filler is at least one of an inorganic pigment and a metal oxide.
Preferably, the protection layer is formed from a water dispersion containing an inorganic pigment and/or a metal oxide dispersed therein and a pH of the water dispersion is 5 or more in terms of a high electric insulating property and a lower probability of an image blur.
Preferably, the inorganic pigment and/or metal oxide is processed with a surface treatment using at least one surface treatment agent in terms of increasing dispersibility, reducing a residual potential at the photoconductor, increasing transparency, preventing a defect in the coat film, imparting wear-resistance, and preventing localized abrasion.
Preferably, the surface treatment agent is at least one of a titanate coupling agent, a higher fatty acid, and/or a metal salt of a higher fatty acid in terms of retaining an insulating property.
Preferably, the inorganic pigment and/or the metal oxide is processed with the surface treatment in an amount of 2 to 30% by weight in terms of achieving the effect of the addition of the filler without increasing the residual potential.
Preferably, the protection layer contains a binder resin containing a resin having an acid value of 10 to 400 (mgKOH/g).
Preferably, the protection layer contains, as a dispersing agent, an organic compound having at least one carboxyl group in a structure thereof and the dispersing agent is a polycarboxylic acid derivative.
Preferably, the dispersing agent is an organic compound having an acid value of 10 to 400 (mgKOH/g).
Preferably, the dispersing agent is added in an amount selected from a range satisfying the following expression:
0.1xe2x89xa6(Amount of Added Dispersing Agentxc3x97Acid Value of Dispersing Agent)/(Amount of Added Filler)xe2x89xa620 
Preferably, a maximum intensity of an electric field applied by the charging means to the charge transporting layer and to the protection layer is xe2x88x9230 V/xcexcm.